Magnetic resonance imaging processes have been known for some time. They are based on the resonance alternating effect between an alternating high-frequency electromagnetic field and specific atomic nuclei of an object to be examined, e.g., a human or an animal body, that is disposed in a strong external magnetic field. The atomic nuclei precess in the magnetic field (B.sub.o) by the so-called Lamor frequency in a manner which is proportional to the strength of the field. When applying an electromagnetic alternating field whose magnetic alternating component (B.sub.1) is vertical to the direction of the strong magnetic field (B.sub.o), the spins of the atomic nuclei flip and connected relaxation times may thus be measured.
In the description of a scientific model, the magnetization of the individual spins is described by total magnetization. This total magnetization in its equilibrium condition is parallel to the external magnetic field and is called equilibrium magnetization. By means of an HF-impulse applied with the Lamor frequency (resonance frequency), the magnetization may be deflected by an angle .alpha. with regard to the direction of the magnetic field. The angle .alpha. is proportional to the time period of the HF-impulse applied and the strength of the magnetic field (B.sub.1) of the HF-impulse. Subsequent to an excitation by the angle .alpha., the total magnetization precesses by the direction of the magnetic field. The precessing magnetization may be recorded by a coil that is oriented vertically to the direction of the magnetic field in form of a voltage signal. The strength of the voltage signal is directly proportional to sin(.alpha.), directly proportional to the density of the spins in the signal emitting volume and inversely proportional to the temperature.
The maximal signal response of a given volume is thus attained after 90.degree. excitation. The recorded signal amplitude decreases exponentially with the relaxation time T.sub.2 *, since the individual spins fall out of phase due to the fluctuating magnetic fields. Simultaneously, the total magnetization increases exponentially again in the direction of the magnetic field towards the equilibrium magnetization with relaxation time T.sub.1. By means of magnetic gradient fields switched at the correct point in time, it is possible to image differentiated combinations from the spin density and the two relaxation times in a gray scale encoded image with spatial resolution.
It is further known to locally induce an amplification of the excitation of the nuclear spins by means of a resonance circuit. For this, so called "fiducial markers" are known that have compartments with special signal-intensive liquids surrounded by a resonance circuit. (Burl et al.: "Tuned Fiducial Markers To Identify Body Locations with Minimal Perturbation of Tissue Magnetization", in: Journal of Magnetic Resonance in Medicine 1996, p. 461-493.) The resonance circuit has the resonance frequency of the magnetic resonance system.
If such a fiducial marker is brought into the imaging volume of a nuclear magnetic resonance tomograph, the resonance circuit is excited when electromagnetic radiation is applied at resonance frequency. This results in the amplification of the magnetic alternating field within the inductance of the resonance circuit. The increased magnetic component of the magnetic field increases the deflection angle .alpha. of the protons within the inductance. With such a small angle of excitation (.alpha.&lt;90.degree.) of the protons by the nuclear spin system, the protons experience an increased excitation angle within the inductance. In the ideal case, protons are excited with a small angle of 1.degree. to 10.degree. in the imaging volume, whereas the protons within the inductance are excited with 90.degree.. Even with identical relaxation times and with an identical spin density, the signal from the compartment surrounding the resonance circuit is clearly more intensive than the signal of the other parts of the image. Since this signal amplification is localized, it may be used for the determination of positions.
According to the law of reciprocity, it is equally true that the magnetic resonance response signals of the protons within the compartment surrounding the resonance circuit (fiducial markers) are amplified. Due to the inductance, the magnetic field lines originating from the spins within the coil are bundled such that more signal is emitted from the volume within the inductance and applied to a corresponding receptor coil. This amplification of emitted and then received signals is considered independent of an increased excitation. Both effects result in a changed signal response of the fiducial marker.
Disadvantageously, fiducial markers make use of separated signal emitting volumes, which for visibility in the magnetic resonance image must be at least a few cubic millimeters in size and which have to be placed specifically in the examination object or which have to be integrated into the systems that are placed in the examination object. Often this is not possible.
With the introduction of open magnets and new techniques with closed magnetic resonance systems it has become possible to carry out interventional and minimally invasive techniques such as punction, catherization and surgical processes under magnetic resonance tomographic control. However, ferromagnetic or paramagnetic metals or impurities in other materials result in artifacts in the images.
Problems result from the tools used for interventional and minimally invasive techniques since they usually consist of ferromagnetic or paramagnetic material and/or that they are so small that they are about the size of one pixel (approximately lmm) in magnetic resonance images. In particular, stents made of metal or plastic are barely visible due to their fine skeletons and can best be located by means of artifacts. When materials that are not visible in the magnetic resonance image are used, they can be seen as "shadows" only. These disadvantages result in the fact that magnetic resonance monitoring is frequently unsatisfactory and that an x-ray process with all its known disadvantages is used instead for imaging.
From DE 195-10-194-A1 an active-invasive magnet resonance system for the production of selective magnetic resonance angiograms is known, whereby an invasive apparatus is provided with a high frequency (HF) coil by which the nuclear spin magnetization of the blood flowing in the vessel is changed locally. By means of special magnetic resonance image impulse sequences, only the blood that has a changed nuclear spin magnetization is selectively detected and imaged.
U.S. Pat. No. 5,445,151 describes a process for flow measurements in flowing fluids, in particular in blood, whereby the invasive apparatus is provided with at least two HF coils, whereby a local change in nuclear spin magnetization produced by one HF coil is sensed at the other HF coil and the delay interval is used for the computation of flow velocity.
The two publications cited do not refer to the imaging of medical apparatus introduced into a body. Furthermore, they have the disadvantage that they are active systems whereby the apparatuses introduced are permanently connected via cable connections to extracorporeal components.
Patent publication DE 195-07-617-A1 describes an magnetic resonance process whereby a surgical instrument, such as a catheter, is introduced into the examination object, whereby the catheter is provided with a micro-coil at its point. The position of the micro-coil is determined by specific sequential techniques.